Piston and barrell assembly with stepped top and hydraulically-actuated fuel injector utilizing same

ABSTRACT

A hydraulically-actuated piston and barrel assembly includes a barrel defining an actuation fluid cavity and a piston bore, which includes an upper bore and a lower bore. A stepped piston is slidably received in the piston bore and moveable between a retracted position and an advanced position. The stepped top of the piston includes a first area that is separate from a second area. The first area and the upper bore define an upper cavity connected to the actuation fluid cavity through a relatively unrestricted flow area, when the piston is in its retracted position. The second area and the lower bore define a lower cavity connected to the actuation fluid cavity through a relatively restricted flow area when the piston is in its retracted position. The first area is exposed to fluid pressure in the upper cavity and the second area is exposed to fluid pressure in the lower cavity over a portion of the piston&#39;s movement from its retracted position toward its advanced position. One of the piston or the barrel, or both, further define a restricted passage that includes a restricted flow area connecting the actuation fluid cavity to the lower cavity. The disclosure also includes a description of a hydraulically-actuated fuel injector that incorporates the piston and barrel assembly described above.

RELATED TO OTHER PATENT APPLICATIONS

This application is a continuation-in-part of co-pending application Ser. No. 08/707,804, filed Aug. 30, 1996, now U.S. Pat. No. 5,687,693, and entitled HYDRAULICALLY-ACTUATED FUEL INJECTOR WITH DIRECT CONTROL NEEDLE VALVE, which is a continuation-in-part of co-pending application Ser. No. 08/668,552, filed Jun. 21, 1996, now U.S. Pat. No. 5,697,342, with the same title, which was a continuation-in-part of application Ser. No. 08/489,660, filed Jun. 12, 1995 and entitled HYDRAULICALLY-ACTUATED FLUID INJECTOR HAVING PRE-INJECTION PRESSURIZABLE FLUID AND DIRECT-OPERATED CHECK, (now U.S. Pat. No. 5,673,669), which was a continuation of application Ser. No. 08/283,232, filed Jul. 29, 1994 with the same title (now U.S. Pat. No. 5,463,996).

TECHNICAL FIELD

The present invention relates generally to hydraulically driven piston and barrel assemblies, and more particularly hydraulically-actuated fuel injectors utilizing stepped piston and barrel assemblies.

BACKGROUND ART

Hydraulically driven piston and barrel assemblies are utilized in diverse ways in a wide variety of related and unrelated machines. In most of these applications, the piston reciprocates in a piston bore defined by the barrel between a retracted position and an advanced position. The piston is driven from its retracted position toward its advanced position by a hydraulic pressure force produced by a pressurized fluid acting on one end of the piston. In some instances, it is desirable to control the initial movement rate of the piston. For example, piston and barrel assemblies are utilized in hydraulically-actuated fuel injectors to pressurize fuel within the injector for each injection event. Over time, engineers have discovered that the injection rate profile can be controlled by controlling the movement rate of the piston. Controlling the initial injection rate is especially important because of the strong influence that initial injection rate shape has on the quality of emissions leaving a particular engine.

Known hydraulically-actuated fuel injection systems and/or components are shown, for example, in U.S. Pat. No. 5,121,730 issued to Ausman et al. on Jun. 16, 1992; U.S. Pat. No. 5,271,371 issued to Meints et al. on Dec. 21, 1993; and, U.S. Pat. No. 5,297,523 issued to Hafner et al. on Mar. 29, 1994. In these hydraulically-actuated fuel injectors, a spring biased needle check opens to commence fuel injection when pressure is raised by an intensifier piston/plunger assembly to a valve opening pressure. The intensifier piston is acted upon by a relatively high pressure actuation fluid, such as engine lubricating oil, when a solenoid driven actuation fluid control valve opens the injector's high pressure inlet. Injection is ended by deactivating the solenoid to release pressure above the intensifier piston. This in turn causes a drop in fuel pressure causing the needle check to close under the action of its return spring and end injection. While these hydraulically-actuated fuel injectors have performed magnificently over many years, there remains room for improvement, especially in the area of shaping an injection rate trace from beginning to end to precisely suit a set of engine operating conditions.

Over the years, engineers have discovered that engine emissions can be significantly reduced at certain operating conditions by providing a particular injection rate trace. In many cases, emissions are improved when the initial injection rate is controllable, and when there is a nearly vertical abrupt end to injection. While these prior hydraulically-actuated fuel injection systems have some ability to control the injection rate shape, there remains room to improve the ability to control the injection rate shape with hydraulically-actuated fuel injection systems. The present invention is intended to improve the ability of hydraulically-actuated fuel injectors to reliably produce better injection rate shapes during each injection event.

DISCLOSURE OF THE INVENTION

A hydraulically-actuated piston and barrel assembly includes a barrel defining an actuation fluid cavity and a piston bore, which includes an upper bore and a lower bore. A piston with a stepped top is slidably received in the piston bore and moveable between a retracted position and an advanced position. The stepped top of the piston includes a first area that is separate from a second area. The first area and the upper bore define an upper cavity connected to the actuation fluid cavity through a relatively unrestricted flow area when the piston is in its retracted position. The second area and the lower bore define a lower cavity connected to the actuation fluid cavity through a relatively restricted flow area when the piston is in its retracted position. The first area is exposed to fluid pressure in the upper cavity and the second area is exposed to fluid pressure in the lower cavity over a portion of the piston's movement from its retracted position toward its advanced position. In one embodiment of the invention, one of the piston or the barrel further defines a restricted passage that includes the restricted flow area connecting the actuation fluid cavity to the lower cavity. In another embodiment of the present invention, the stepped top of the piston defines a slot positioned in the upper bore when the piston is in its retracted position. In both embodiments the particular geometry of the piston bore and piston cause the initial movement rate of the piston to be relatively slow.

In another embodiment of the present invention a hydraulically-actuated fuel injector includes an injector body, a plunger bore, a nozzle chamber, a nozzle outlet that opens to the nozzle chamber, and a piston and barrel assembly of the type just described. The plunger is slidably positioned in the plunger bore, and a portion of the plunger and the plunger bore define a fuel pressurization chamber that opens to the nozzle chamber. A needle valve member is positioned in the nozzle chamber and moveable between an open position in which the nozzle outlet is open and a closed position in which the nozzle outlet is blocked.

In still another embodiment of the present invention a hydraulically-actuated fuel injector includes an injector body that defines a nozzle chamber, a needle control chamber, and a nozzle outlet that opens to the nozzle chamber, as well as a piston and barrel assembly of the type previously described. The needle valve member is positioned in the nozzle chamber and moveable between an open position in which the nozzle outlet is open and a closed position which the nozzle outlet is blocked. The needle valve member also includes a closing hydraulic surface exposed to pressure in the needle control chamber. The needle control valve is attached to the injector body and is moveable between an off position in which the needle control chamber is opened to a source of high pressure fluid and an on position in which the needle control chamber is opened to a low pressure passage. One object of the present invention is to improve the ability to control the initial movement rate of a hydraulically driven piston in a piston and barrel assembly.

Another object of the present invention is to control the initial injection rate shape of a hydraulically-actuated fuel injector by controlling the initial movement rate of the injector's piston.

Still another object of the present invention is to improve hydraulically-actuated fuel injectors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectioned side elevational view of a hydraulically-actuated fuel injector according to the present invention.

FIG. 2 is a partial sectioned side elevational view of the piston area portion of the fuel injector shown in FIG. 1.

FIGS. 3a-3d are a group of curves showing component positions and injection parameters versus time over a single "boot shaped-square" injection event.

FIGS. 4a-4d are a group of curves showing component positions and injection parameters versus time over a single "ramp-square" injection event.

FIG. 5 is a partial sectioned side elevational view of a piston and barrel assembly according to another embodiment of the present invention.

FIG. 6 is a partial sectioned side elevational view of still another piston and barrel assembly according to the present invention.

FIG. 7a is a partial sectioned side elevational view of another piston and barrel assembly according to the present invention.

FIG. 7b is a top elevational view of the inner portion of the stepped piston shown in FIG. 7a.

FIG. 8 is a partial sectioned side elevational view of a piston and barrel assembly according to another embodiment of the present invention.

FIG. 9 is a partial sectioned side elevational view of still another piston and barrel assembly according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIG. 1, a hydraulically-actuated fuel injector 10 utilizes a single solenoid 15 to control both the flow of high pressure actuation fluid into the injector and the flow of high pressure fuel out of the injector. Injector 10 includes an injector body 11 made up of various components attached together in a manner known in the art. The injector includes a hydraulic means for pressurizing fuel that includes an actuation fluid control valve that alternately opens actuation fluid cavity 22 to the high pressure of actuation fluid inlet 20 or the low pressure of actuation fluid drain 21. The actuation fluid control valve includes two-way solenoid 15, which is attached to a pin 16 and biased toward a retracted position by a spring 17. The actuation fluid control valve also includes a ball valve member 55, and a spool valve member 60. Ball valve member 55 is positioned between a high pressure seat 56 and a low pressure seat 57. When solenoid 15 is deactivated, high pressure actuation fluid acting on ball valve member 55 holds the same in low pressure seat 57 to close actuation fluid drain 26. When solenoid 15 is activated, pin 16 moves downward contacting ball valve member 55 and pushing it downward to close high pressure seat 56 and open low pressure seat 57.

Spool valve member 60 reciprocates in a spool bore between a first position (as shown) in which actuation fluid cavity 22 is open to low pressure actuation fluid drain 21, and a second lower position in which drain 21 is closed but actuation fluid cavity 22 is open to high pressure actuation fluid inlet 20 via radial openings 61 in the spool valve member. Spool valve member 60 is biased toward its first position by a compression spring 64. When solenoid 15 is energized to push ball valve member 55 to close high pressure seat 56 and open low pressure seat 57, spool hydraulic surface 62 becomes exposed to the low pressure in drain 26 via spool control passage 29 and control passage 28. When this occurs, spool valve member 60 becomes hydraulically imbalanced and moves downward toward its second position against the action of biasing spring 64.

The hydraulic means for pressurizing fuel further includes a stepped piston 80 which is slidably received in a piston bore 70 and moveable between a retracted position (as shown) and an advanced position. A plunger 53 is in contact with the underside of piston 80 and is slidably positioned in a plunger bore 52. Plunger 53 and stepped piston 80 are biased toward their retracted position by a return spring 54. Finally, a portion of plunger bore 52 and plunger 53 define a fuel pressurization chamber 37 in which fuel is pressurized to injection pressure when piston 80 and plunger 53 undergo their downward stroke.

Fuel enters injector 10 through a fuel inlet 35 and travels upward through fuel supply passage 36 past ball check 43 and into fuel pressurization chamber 37 when plunger 53 and piston 80 are undergoing their return stroke. Ball check 43 prevents the back flow of fuel when plunger 53 is undergoing its downward pumping stroke during an injection event.

Fuel pressurization chamber 37 communicates with nozzle chamber 39 via a nozzle supply passage 38. Nozzle chamber 39 opens to nozzle outlet 40. A needle valve member 44 is positioned in nozzle chamber 39 and moveable between an open position in which nozzle outlet 40 is open and a closed position in which the nozzle outlet is blocked to nozzle chamber 39. Needle valve member is actually an assembly of component parts including a needle 45, a disk spacer 46, a pin spacer 47 and a needle piston 48. Needle piston 48 includes a closing hydraulic surface 49 exposed to fluid pressure in a needle control chamber 31. Needle control chamber 31 communicates with control passage 28 via a needle control passage 30. When solenoid 15 is deactivated, closing hydraulic surface 49 of needle valve member 44 is exposed to the high pressure of actuation fluid inlet 20 via needle control passage 30, control passage 28, past high pressure seat 56 and through radial openings 61 in spool valve member 60. When solenoid 15 is energized to close high pressure seat 56, needle control chamber 31 is exposed to the low pressure of drain 26 via needle control passage 30, control passage 28 and past low pressure seat 57. This aspect of the invention provides direct control over needle valve member 44. In other words, needle valve member 44 can be held in its closed position by exposing closing hydraulic surface 49 to the high pressure of actuation fluid inlet. Needle valve member can only move to its open position when closing hydraulic surface 49 is exposed to the low pressure of drain 26. And then, needle valve member 44 can only open when fuel pressure within nozzle chamber 39 acting on lifting hydraulic surface 42 is sufficient to overcome the action of biasing spring 50. This direct control aspect of the invention allows for split injections and an abrupt end to injection as more thoroughly discussed and described in the parent application identified above.

In order to prevent secondary injections and vent fluid pressure from actuation fluid cavity 22 and piston bore 70 toward the end of an injection event, injector body 11 also defines a pressure relief passage 32 that opens to a third drain 27. A relief ball 67 is held in place to close relief passage 32 during an injection event by the downward force provided by spool valve member 60 as transmitted through pin 66. At the end of an injection event, solenoid 15 is deactivated to reopen high pressure seat 56. This resumes high pressure actuation fluid on spool hydraulic surface 62 causing it to move upward. At the same time, residual hydraulic actuation fluid pressure acting on relief ball 67 opens relief passage 32 to low pressure drain 27. At the same time, the movement of relief ball 67 provides a boost to hasten the movement of spool valve member 60 in its upward travel via the contact between the two by pin 66. This feature of the invention prevents secondary injections which might otherwise occur due to pressure spikes created within the injector when the needle valve member is abruptly closed at the end of an injection event.

Referring now to FIG. 2, the area in and around the stepped top of piston 80 is illustrated. Piston bore 70 includes an upper bore 72 and a larger diameter lower bore 71. The stepped top of piston 80 includes a first area 81 that is separated from a second area 82 by a regular cylindrical portion 84. First area 81 and upper bore 72 define an upper cavity 90 that is connected to actuation fluid cavity 22 through a relatively unrestricted flow area 23 when piston 80 is in its retracted position, as shown. Second area 82 and lower bore 71 define a lower cavity 91 that is connected to the actuation fluid cavity 22 via a restricted passage 24 that includes a restricted flow area 25, when the piston is in its retracted position. When the piston begins its movement from its retracted position toward its advanced position, the first area 81 is exposed to the full fluid pressure in upper cavity 90, whereas second area 82 is exposed to the fluid pressure in lower cavity 91. Because of the rate at which the volume above second area 82 grows as the piston 80 moves in its downward stroke, the restricted flow area 25 prevents second area 82 from experiencing the full fluid pressure in actuation fluid cavity 22 until the piston moves a sufficient distance downward that fluid can also flow around annular taper 85 onto second area 82. In this embodiment, restricted passage 24 is defined by barrel 12.

Also shown in FIG. 2 are the design parameters "A", "B", "C", and "D". The height of annular taper 85 is preferably chosen to be sufficiently long that the movement rate of the piston is not influenced by the height of the annular taper. This eliminates one possible area of variability when injectors of this type are mass produced. Control over the design parameters A, B, C and D gives one substantial control over the initial movement rate of piston 80, and hence the initial injection rate profile from the injector. The hole diameter "A", which defines a restrictive flow area, and the diameter "B" and the height "C" of the regular cylindrical portion can be sized such that when the regular cylindrical portion 84 is still in upper bore 72, the fluid pressure in lower cavity 91 can be made to be essentially constant as shown in FIG. 3b. Thus, the height of regular cylindrical portion 84 controls the duration of the "flat portion" 97 of the boot injection profile illustrated in FIG. 3d. As the piston 80 continues its downward movement, the regular cylindrical portion 84 moves out of upper bore 72 to open an annular gap between annular taper 84 and upper bore 72. This allows actuation fluid to flow into lower cavity 91 both through restricted passage 24 and past annular taper 85 so that pressure in lower cavity 91 begins to rise. As a result, fuel pressure increases, producing the ramp up portion 98 shown in FIG. 3d. The slope "D" of annular taper 85 controls the slope of the ramp up portion 98.

The height "C" of regular cylindrical portion 84 controls the duration of the initial flat portion 98 of the boot injection. If dimension "C" is short enough, the initial flat portion would disappear, resulting in a ramp up only portion 99 as illustrated in FIG. 4d. Still, dimension "C" preferably has some minimal lead distance length because some movement of the piston is typically necessary to compress the fuel below plunger 53 to a satisfactory injection pressure. Thus, by varying dimensions "A", "B", "C", and "D", the present invention provides near total flexibility in controlling the front portion of the injection rate trace, which is very important in controlling engine emissions.

Referring now to FIG. 5, an alternative embodiment of the present invention is shown which includes a piston 180 with a stepped top slidably received in a piston bore 170, which includes a lower bore 171 and an upper bore 172. Like the earlier embodiment, stepped piston 180 includes a first area 181 that is separated from a second area 182 by a regular cylindrical portion 184. Stepped piston 180 sits atop a plunger 53 and a return spring 54, which are identical to the embodiment previously described.

Like the previous embodiment, the first area 181 and upper bore 172 define an upper cavity 190 that is connected to an actuation fluid cavity 122 through a relatively unrestrictive flow area. The second area 182 and the lower bore 171 define a lower cavity connected to actuation fluid cavity 122 through a relatively restricted flow passage 124 defined by the area between regular cylindrical portion 184 and upper bore 172. This version performs substantially similar to the earlier version but instead of the barrel defining a separate restricted passageway, the piston and barrel define restricted passage 124. Also, this embodiment is different in that instead of an annular taper on the upper stepped portion of the piston, a slot 187 is machined therein. In this case, the width of slot 187 is the counterpart to the slope "D" shown in FIG. 2. In other words, the wider the slot, the steeper the ramp up portion of the injection profile. In this embodiment, the difference in the height of the upper step portion from the depth of the slot corresponds to the dimension "C" shown in FIG. 2. In other words, the deeper the slot the less a flat portion 97 (FIG. 3d) will appear in the injection rate profile.

Referring now to FIG. 6, still another embodiment of the present invention is shown in which the piston itself defines the restricted passage to 224. Like the previous embodiments a stepped piston 280 is slidably received in a piston bore 270, which includes a lower bore 271 and an upper bore 272. A first area 281 is separated from a second area 282 by a regular cylindrical portion 284. The first area 281 and upper bore 272 define an upper cavity 290 that is open to the actuation fluid cavity 222 via a relatively unrestricted flow area. Like the previous embodiments, the second area 282 and the lower bore 271 define a lower cavity 291 that is connected to actuation fluid cavity 222 via a restricted passage 224. Like the embodiment shown in FIG. 2, regular cylindrical portion 284 substantially isolates the lower cavity from the upper cavity. This embodiment of the invention operates substantially identical to the earlier embodiments described, but just contains different geometry to accomplish the same purposes.

Referring now to FIG. 7a, still another embodiment is shown in which a stepped plunger 380 is slidably received and a piston bore 370 that includes a lower bore 371 and an upper bore 372. A first area 381 and the upper bore 372 define an upper cavity, as in the previous embodiments. Likewise, a second area 382 and lower bore 371 define a lower cavity that is connected to actuation fluid cavity 322 via a restricted passage 324, which in this embodiment is created by slots cut into annular taper 385. Thus, in this embodiment like the embodiment shown in FIG. 5, the piston and plunger define the restricted passage 324. However, this embodiment is like the embodiment shown in FIG. 1 in that it includes a regular cylindrical portion 384 and an annular taper portion 385.

Referring now to FIG. 8, another embodiment of the present invention is shown that behaves identical to the previous embodiments but includes different geometry. In this case, the second area 482 is located inside of the first area 481. Like the previous embodiments, a restricted passage 424 opens into a first cavity 491. A relatively unrestricted flow area 423 opens to a second cavity 490. Like the previous embodiments, a stepped piston 480 is slidably received in a piston bore 470 that includes an upper bore 471 and a lower bore 472. Also like the embodiment shown in FIG. 2, the stepped piston includes a regular cylindrical portion 484 and an annular taper 485.

FIG. 9 shows another embodiment of the present invention in which still another geometrical variation of the present invention is shown. In particular, a stepped piston 580 is slidably received in a piston bore 570. The first area 581 is separated from a second area 582 by a regular cylindrical portion 584. Like the previous embodiments, a first cavity 591 is connected to an actuation fluid cavity (not shown) through a restricted passage 524 that includes a restricted flow area 525. Also like the previous embodiment, a second cavity 590, which acts upon first area 581, is connected to an actuation fluid cavity via an unrestricted flow area 523.

Industrial Applicability

The present invention finds potential application to any piston and barrel assembly that is hydraulically driven and in which it is desirable to slow the initial movement rate of the piston. This slowing of the initial movement rate of the piston is accomplished by machining various geometrical relationships between the piston and the piston bore rather than through control of the pressure of the fluid acting on the piston as a whole. The present invention finds special application in the case of hydraulically-actuated fuel injectors in which it is desirable to slow the initial movement rate of the piston in order to provide a more desirable front end injection rate trace to reduce undesirable engine emissions.

While any of the embodiments illustrated could be utilized in a fuel injector, the embodiment shown in FIG. 1 and 2 is most desired because of the ease with which circular features can be machined in a bore or on a cylindrical piston to relatively tight tolerances. In other words, the slots illustrated in some of the embodiments could prove more difficult to reliably manufacture is mass quantities while maintaining the tight dimension tolerances necessary to produce consistent results.

In any event, the above description is intended for illustrative purposes only and is not intended to limit the scope of the present invention in any way. In other words, the various geometrically shaped piston and barrel assemblies illustrated above are not intended as an exhaustive presentation of examples which would fall within the scope of the present invention. Those skilled in the art will appreciate that other piston and barrel assembly geometries, which are not shown, will fall within the scope of the present invention. Other aspects, objects, and advantages of this invention can be obtained from a study of the drawings, the disclosure, and the appended claims. 

We claim:
 1. A hydraulically actuated piston and barrel assembly comprising:a barrel defining an actuation fluid cavity and a piston bore, which includes an upper bore and a lower bore; a piston with a stepped top being slidably received in said piston bore and moveable between a retracted position and an advanced position; said stepped top of said piston including a first area that is separate from a second area; said first area and said upper bore defining an upper cavity connected to said actuation fluid cavity through a relatively unrestricted flow area when said piston is in said retracted position; said second area and said lower bore defining a lower cavity connected to said actuation fluid cavity through a relatively restricted flow area when said piston is in said retracted position; said first area being exposed to fluid pressure in said upper cavity and said second area being exposed to fluid pressure in said lower cavity when said piston begins moving from said retracted position toward said advanced position; and one of said piston or said barrel further defining a restricted passage connecting said actuation fluid cavity to said lower cavity, and said restricted passage includes said restricted flow area.
 2. The hydraulically actuated piston and barrel assembly of claim 1 wherein said first area is separated from said second area by a regular cylindrical portion.
 3. The hydraulically actuated piston and barrel assembly of claim 2 wherein said regular cylindrical portion substantially isolates said lower cavity from said upper cavity when said piston is in said retracted position.
 4. The hydraulically actuated piston and barrel assembly of claim 3 wherein said first area includes an annular taper adjacent said regular cylindrical portion.
 5. The hydraulically actuated piston and barrel assembly of claim 4 wherein said barrel defines said restricted passage.
 6. The hydraulically actuated piston and barrel assembly of claim 4 wherein said piston defines said restricted passage.
 7. A hydraulically actuated piston and barrel assembly comprising:a barrel defining an actuation fluid cavity and a piston bore, which includes an upper bore and a lower bore; a piston with a stepped top being slidably received in said piston bore and moveable between a retracted position and an advanced position; said stepped top of said piston including a first area that is separate from a second area; said first area and said upper bore defining an upper cavity connected to said actuation fluid cavity through a relatively unrestricted flow area when said piston is in said retracted position; said second area and said lower bore defining a lower cavity connected to said actuation fluid cavity through a relatively restricted flow area when said piston is in said retracted position; said first area being exposed to fluid pressure in said upper cavity and said second area being exposed to fluid pressure in said lower cavity over a portion of said piston's movement from said retracted position toward said advanced position; and said stepped top of said piston defining a slot positioned in said upper bore when said piston is in said retracted position.
 8. The hydraulically actuated piston and barrel assembly of claim 7 wherein said piston and said barrel defining a restricted passage connecting said actuation fluid cavity to said lower cavity, and said restricted passage includes said restricted flow area.
 9. A hydraulically actuated fuel injector comprising:an injector body that includes a barrel and defines a plunger bore, a nozzle chamber and a nozzle outlet that opens to said nozzle chamber; a barrel defining an actuation fluid cavity and a piston bore, which includes an upper bore and a lower bore; a piston with a stepped top being slidably received in said piston bore and moveable between a retracted position and an advanced position; said stepped top of said piston including a first area that is separate from a second area; said first area and said upper bore defining an upper cavity connected to said actuation fluid cavity through a relatively unrestricted flow area when said piston is in said retracted position; said second area and said lower bore defining a lower cavity connected to said actuation fluid cavity through a relatively restricted flow area when said piston is in said retracted position; and said first area being exposed to fluid pressure in said upper cavity and said second area being exposed to fluid pressure in said lower cavity over a portion of said piston's movement from said retracted position toward said advanced position; one of said piston or said barrel further defining a restricted passage connecting said actuation fluid cavity to said lower cavity, and said restricted passage includes said restricted flow area; a plunger slidably positioned in said plunger bore; a portion of said plunger and said plunger bore defining a fuel pressurization chamber that opens to said nozzle chamber; a needle valve member positioned in said nozzle chamber and moveable between an open position in which said nozzle outlet is open and a closed position in which said nozzle outlet is blocked.
 10. The hydraulically actuated fuel injector of claim 9 wherein said first area is separated from said second area by a regular cylindrical portion.
 11. The hydraulically actuated fuel injector of claim 10 wherein said barrel defines said restricted passage; andsaid cylindrical portion substantially isolating said lower cavity from said upper cavity when said piston is in said retracted position.
 12. The hydraulically actuated fuel injector of claim 11 wherein said first area includes an annular taper adjacent said regular cylindrical portion.
 13. A hydraulically actuated fuel injector comprising:an injector body that includes a barrel and defines a plunger bore, a nozzle chamber and a nozzle outlet that opens to said nozzle chamber; a barrel defining an actuation fluid cavity and a piston bore, which includes an upper bore and a lower bore; a piston with a stepped top being slidably received in said piston bore and moveable between a retracted position and an advanced position; said stepped top of said piston including a first area that is separate from a second area; said first area and said upper bore defining an upper cavity connected to said actuation fluid cavity through a relatively unrestricted flow area when said piston is in said retracted position; said second area and said lower bore defining a lower cavity connected to said actuation fluid cavity through a relatively restricted flow area when said piston is in said retracted position; and said first area being exposed to fluid pressure in said upper cavity and said second area being exposed to fluid pressure in said lower cavity over a portion of said piston's movement from said retracted position toward said advanced position; said stepped top of said piston defining a slot positioned in said upper bore when said piston is in said retracted position; a plunger slidably positioned in said plunger bore; a portion of said plunger and said plunger bore defining a fuel pressurization chamber that opens to said nozzle chamber; a needle valve member positioned in said nozzle chamber and moveable between an open position in which said nozzle outlet is open and a closed position in which said nozzle outlet is blocked.
 14. The hydraulically actuated fuel injector of claim 13 wherein said first area is separated from said second area by a regular cylindrical portion.
 15. The hydraulically actuated fuel injector of claim 11 wherein said regular cylindrical portion and said upper bore define a restricted flow passage that includes said restricted flow area.
 16. A hydraulically actuated fuel injector comprising:an injector body that includes a barrel and defines a nozzle chamber, a needle control chamber and a nozzle outlet that opens to said nozzle chamber; a barrel defining an actuation fluid cavity and a piston bore, which includes an upper bore and a lower bore; hydraulic means, within said injector body for pressurizing fuel in said nozzle chamber, said hydraulic means including a piston with a stepped top being slidably received in said piston bore and moveable between a retracted position and an advanced position; said stepped top of said piston including a first area that is separate from a second area; said first area and said upper bore defining an upper cavity connected to said actuation fluid cavity through a relatively unrestricted flow area when said piston is in said retracted position; said second area and said lower bore defining a lower cavity connected to said actuation fluid cavity through a relatively restricted flow area when said piston is in said retracted position; and said first area being exposed to fluid pressure in said upper cavity and said second area being exposed to fluid pressure in said lower cavity over a portion of said piston's movement from said retracted position toward said advanced position; a needle valve member positioned in said nozzle chamber and moveable between an open position in which said nozzle outlet is open and a closed position in which said nozzle outlet is blocked, and said needle valve member including a closing hydraulic surface exposed to pressure in said needle control chamber; and a needle control valve attached to said injector body and being moveable between an off position in which said needle control chamber is opened to a source of high pressure fluid and an on position in which said needle control chamber is opened to a low pressure passage.
 17. The hydraulically actuated fuel injector of claim 16 wherein said injector body includes an actuation fluid inlet and an actuation fluid drain;and further comprising an actuation fluid control valve having a solenoid and being moveable between a first position in which said actuation fluid inlet is open to said actuation fluid cavity and a second position in which said actuation fluid inlet is closed to said actuation fluid cavity; and said needle control valve further includes said solenoid.
 18. The hydraulically actuated fuel injector of claim 17 wherein said source of high pressure fluid is said actuation fluid inlet; andsaid low pressure passage is said actuation fluid drain.
 19. The hydraulically actuated fuel injector of claim 18 wherein said needle valve member includes an opening hydraulic surface exposed to pressure in said nozzle chamber; andsaid closing hydraulic surface and said opening hydraulic surface are sized and arranged such that said needle valve member is hydraulically biased toward said closed position when said needle control chamber is opened to said source of high pressure fluid.
 20. The hydraulically actuated fuel injector of claim 19 wherein said actuation fluid inlet is isolated from said nozzle chamber. 